1. Field of the Invention
The present invention relates to a laser diffraction-type particle size distribution measuring apparatus and method and more particularly, an apparatus and method for determining the stability of the measured results.
2. Description of Related Art
The necessity to measure accurately the size and distribution of particles has become increasingly important in order to determine and evaluate the performance of various powder-type substances across a wide field of applications, such as pharmaceuticals, food stuffs, ceramics, cosmetics, paints, and coloring matters. One form of the apparatus and method utilized to determine particle distribution is a laser diffraction method wherein a powdery sample is disbursed and stirred in a carrier liquid called a dispersion medium to form a suspension. This resulting suspension is then introduced into a measurement flow cell that is then illuminated with laser beams. The detected scattered light from impact with the particles, is measured by an appropriate optical detector array and the results of this measurement are then used to calculate the intensity distribution of the resulting diffracted light and the particle distribution of the sample on the basis of Fraunhofer's diffraction theory and Mie's scattering theory.
Conventionally, the operation of such apparatus has required that the suspension be circulated through a suspension circulating system to reach a uniform condition of suspension so that the flow cell will be appropriately charged with a sample that is representative of the specimen being tested. In order to determine whether the sample has reached a state in which accurate measurements can be made, the suspension is usually circulated through a circulating system after a suitable time period, for example, four or five minutes, after an operation of an ultrasonic vibrator adequately dispersed the particle sample in the dispersion medium. Subsequently, the flow cell is illuminated with laser beams to provide data to create a particle-size distribution graph, for example, of the type shown on the screen in FIG. 4, so that it can be compared with a standard particle-size distribution graph and when there is substantially no difference between them, it can be judged that the suspension is in a stable or measurement condition.
Another method that has been utilized is to again operate the ultrasonic vibrator for a set period of time and then conduct two measurement cycles to obtain two particle-size distribution graphs. These graphs can then be physically compared and when it is decided that there is no significant difference between both particle-size distribution graphs, it is possible to conduct the measurement cycle. As can be appreciated, in the former method it is necessary that a plurality of different kinds of standard particle-size distribution graphs must be prepared and stored so that they can be suitably selected and compared with an obtained particle-size distribution graph. As can be reasonably expected, this may require numerous comparisons until the operator can decide that the suspension of the sample has arrived at a stable condition or measurement condition.
In the latter method mentioned above, it is not required to prepare a plurality of standard particle-size distribution graphs and store them, but it is required to obtain the particle-size distribution graph numerous times until the suspension arrives at a stationary condition and to make individual comparison from the results of each measurement. A printer can print out the results to facilitate such comparisons.
In both operations of a conventional laser diffraction-type particle-size distribution measuring method, the operator involvement can be time consuming and can lead to errors.
As can be appreciated, these problems have occurred not only in those methods wherein the suspension sample is ultrasonically dispersed in a dispersing bath and circulated between the dispersing bath and a flow cell, but also where the suspension is merely stirred in the bath and circulated, and also in the use of a so-called batch method in which the suspension is not circulated and is only ultrasonically dispersed.
The prior art is accordingly looking for an improved method and apparatus for determining the stability and uniform suspension of the particles to enable a measurement cycle.